This invention relates to an improved method for setting calcined gypsum. More specifically, it relates to the use of ground gypsum and bisulfate salts to reduce the set time, decrease the water demand and reduce the drying time when making gypsum products.
Gypsum is a very practical and cost effective building material. It is also known as calcium sulfate dihydrate. When mined calcium sulfate dihydrate has been ground, it is also referred to as landplaster. Calcium sulfate hemihydrate is also known as calcined gypsum, stucco, calcium sulfate semihydrate, calcium sulfate half-hydrate or plaster of Paris. Synthetic gypsum, which is a byproduct of flue gas desulfurization processes from power plants, may also be used. When it is mined, raw gypsum is found in the dihydrate form. In this form, there are two water molecules of water associated with each molecule of calcium sulfate. In order to produce the hemihydrate form, the gypsum can be calcined to drive off some of the water of hydration by the following equation:
CaSO4.2H2Oxe2x86x92CaSO4.1/2H2O+3/2H2O
A number of usefull gypsum products can be made by mixing the calcium sulfate hemihydrate with water to form a slurry and casting the resulting product slurry into the desired shape. The product slurry is permitted to set by allowing the calcium sulfate hemihydrate to react with sufficient water to convert the hemihydrate into a matrix of interlocking dihydrate crystals. As the crystal matrix forms, the product slurry becomes firm and holds the desired shape. Excess water must then be removed from the product by drying.
In the process for making gypsum products, the setting and drying steps are the most intensive in terms of time and energy. The setting time of the slurry depends on a number of factors, including the age of the calcined gypsum, impurities in the calcined gypsum, surface area, pH, particle size and the temperature at the time of mixing. The use of additives or process conditions that reduce the drying time of the gypsum products lead to lower costs. The lower energy costs are a result of lower temperatures or less drying time used in the drying stage. Another additive benefit would be the ability to increase the speed of production line, yielding more product with the same resources. Decreases in the amount of time required to set the product would lead to similar reduction in production costs.
Many additives are known to accelerate the hydration reaction. In the late 1950""s the Bureau of Mines investigated the effect of dissolved substances on the set time of gypsum plasters. At that time the Bureau tested many substances that are impurities in either the gypsum or the water to determine if they caused any accelerative or retardive effects on hydration. Sulfates, including bisulfates, and gypsum were determined to be accelerators at that time. (xe2x80x9cHydration-Rate Studies of Gypsum Plasters: Effects of Small Amounts of Dissolved Substancesxe2x80x9d J. P. Coughlin, K. C. Conway, M. F. Koehler and D. F. Barry, Bureau of Mines Report of Investigations 5477, 1959.)
U.S. Pat. No. 2,216,555 (xe2x80x9cKingxe2x80x9d) teaches that acid-reacting soluble sulfates, including bisulfates, are known to accelerate. However, acidic sulfates also tend to react with impurities in the raw gypsum, particularly carbonates. King solves this problem by adding lime and set retarders to the plaster to control these decomposition reactions with carbonates. However, plasters containing an excess of lime form a thin, soft skin of unset plaster on the surface of the gypsum product which may lead to surface imperfections.
Ground gypsum has also been known as a set accelerator, particularly when combined with a coating such as a starch, as in U.S. Pat. No. 3,870,538, or a sugar, as in U.S. Pat. No. 4,681,644. It is thought that the addition of the dihydrate serves as xe2x80x9cseed crystals,xe2x80x9d or nucleation sites promoting faster growth of the dihydrate crystals in the hydrating product slurry. Gypsum is very active when freshly ground, but upon aging its effectiveness decreases rapidly. Thus, it is preferable to provide a coating for the gypsum, that will reduce deactivation of the material over time.
The speed at which the gypsum products may be dried is determined by two factors. Water demand is the amount of water needed to produce a slurry of a certain fluidity. It can be changed by the use of rheology modifying additives to the gypsum slurry. Additives that increase the water demand will lengthen the drying time of the gypsum products. Even where the same amount of water is added to the slurry, additives may also change the rate at which excess water is driven off from the formed product. As the excess water evaporates, capillary action draws water from the interior of the crystal matrix toward the surface of the product. When the water is hindered from moving to the surface, the drying process is slowed. If an additive increases water demand or decreases the rate at which excess water evaporates from the product, it may result in the need to slow the production line, increase kiln exposure time, or incur greater energy costs from higher kiln temperatures. Any of these alternatives leads to higher production costs for the gypsum products.
It is an object of this invention to provide a more economical method for making set gypsum articles by reducing production costs.
It is another object of this invention to provide an improved accelerator for increasing the rate at which calcined gypsum is set.
It is still another object of this invention to reduce the cost of drying gypsum articles by reducing the water demand of the slurry.
It is yet another object of this invention to provide an economical method of making set gypsum products that release excess water more easily and dry faster.
The above-listed objects are met or exceeded by the present additive composition and process for making set gypsum products, which features a novel system of additives to significantly reduce the time and energy required to hydrate and dry gypsum products.
More specifically, the present invention provides additives for accelerating the setting reaction of calcined gypsum comprising a gypsum accelerator combined with a bisulfate salt. The gypsum accelerator comprises ground calcium sulfate dihydrate. In a preferred embodiment the gypsum accelerator is treated with a coating agent such as a starch, a sugar, and/or boric acid. In the preferred embodiment, the additives are used in a process in which the bisulfate salt and a portion of the calcium sulfate hemihydrate are prewet prior to mixing in a main mixer.
It has been discovered that when bisulfate salts are used together with a gypsum accelerator a surprising effect is observed. The combination of the two ingredients gives a faster set rate than when either of the two additives are used separately. Further, when used with the gypsum accelerator, there have been no significant reductions in the strength of the product as is seen with some additives in the prior art. Faster set of the product results in more efficient product manufacture. When the product hydrates more quickly, the speed of the production line may be increased, making more product in the same time period. If production facilities are being newly built or redesigned, the line could be made smaller for a given production rate, allowing the space saved to be utilized for another purpose.
This combination of additives also provides additional benefits to the process of setting calcined gypsum that have not been previously observed in the prior art with these additives individually. One significant additional benefit is a reduction in the amount of energy required to dry the product. The additives of the present invention decrease drying time in two ways. First, less water is needed to form the slurry because the flowability of the slurry is improved. This results in less excess water present in the product after the hydration reaction is complete. In addition, excess water that is present is more easily removed. Thus, the same amount of product can be made by reducing the temperature of the drying kiln, yielding a direct savings in energy costs. Speed of the production lines can be increased, making more product for the same amount of fuel consumed. Either of these methods, or a combination of both of them, may be used to reduce the energy costs per unit of product.
It is also believed that the additives of the present invention reduce scaling and prevent the build up of calcium deposits in the mixing equipment. Where equipment stays cleaner longer, there is less down time on the line to clean and service it. Each of these benefits of the present invention can lead to a more cost effective means of producing gypsum products.